The present invention generally relates to micromachined devices, and particularly microelectromechanical system (MEMS) devices formed by deep reactive ion etching (DRIE) processes. More particularly, this invention relates to structures and methods for improving yields and device reliability of MEMS devices formed by DRIE processes.
A wide variety of MEMS devices are known, including accelerometers, rate sensors, actuators, motors, microfluidic mixing devices, springs for optical-moving mirrors, etc. As an example, rotational accelerometers that employ MEMS devices are widely used in computer disk drive read/write heads to compensate for the effects of vibration and shock. Other applications for rotational accelerometers that use MEMS devices include VCR cameras and aerospace and automotive safety control systems and navigational systems. Rotational rate sensors and accelerometers have been developed whose MEMS devices are fabricated in a semiconductor chip. Notable examples of rotational rate sensors include a plated metal sensing ring disclosed in U.S. Pat. No. 5,450,751 to Putty et al., and electrically-conductive, micromachined silicon sensing rings disclosed in U.S. Pat. No. 5,547,093 to Sparks and U.S. Pat. No. 5,872,313 to Zarabadi et al., all of which are assigned to the assignee of this invention. U.S. Pat. No. 6,257,062 to Rich, also assigned to the assignee of this invention and incorporated herein by reference, discloses a MEMS device that employs a disk-shaped semiconductor proof mass for sensing rotational acceleration. Rich""s proof mass is suspended above a cavity by a number of tethers that extend from the perimeter of the proof mass to the rim of a substrate surrounding the proof mass. The tethers allow the proof mass to rotate about an axis perpendicular to the plane containing the proof mass and tethers. Fingers extend radially outward from the proof mass and are interdigitized with fingers extending radially inward from the substrate rim. Pairs of the cantilevered fingers of the proof mass and rim are capacitively coupled to produce an output signal that varies as a function of the distances between adjacent paired fingers, which in turn vary with the angular position of the proof mass as it rotates about its axis of rotation.
Sensors of the type described above are capable of extremely precise measurements, and are therefore desirable for use in a wide variety of applications. However, the intricate proof masses and associated sensing structures required for such sensors must be precisely formed in order to ensure the proper operation of the sensor. For example, Rich""s device requires a sufficient gap between paired interdigitized fingers to prevent stiction and shorting, yet paired fingers must also be sufficiently close to produce a sufficient capacitive output signal for the sensor. Increasing the area of the fingers to achieve greater capacitive coupling increases the capacitive output for a given finger gap. However, traditional etching techniques have not generally been well suited for mass-producing semiconductor micromachines with high aspect ratios necessary to etch closely-spaced fingers in a relatively thick substrate. For example, with conventional etching techniques it is difficult to achieve a 10:1 aspect ratio capable of forming interdigitized fingers spaced three micrometers apart in a silicon layer that is thirty micrometers thick. In addition to operational considerations, there is a continuing emphasis for MEMS devices that are lower in cost, which is strongly impacted by process yield, yet exhibit high reliability and performance capability. Consequently, improvements in the processing of MEMS devices for sensing and other applications are highly desirable.
Deep reactive ion etching (DRIE) is a known process capable of performing deep, high aspect ratio anisotropic etches of silicon and polysilicon, and is therefore desirable for producing semiconductor MEMS of the type taught by Rich. However, DRIE is a young technology practiced largely for research and development. Accordingly, to take advantage of the unique capabilities of the DRIE process, its etch idiosyncrasies must be determined and reconciled to render it suitable for high volume manufacturing. In practice, a difficulty of micromachining Rich""s MEMS using the DRIE process has been that, even with individual etch times calculated for each semiconductor wafer, it is difficult to not overetch or underetch certain features in the wafer. Overetching a wafer typically causes significant damage to the proof mass fingers and can render the device nonfunctional, leading to a significant reduction in wafer yield. On the other hand, underetching causes undesired electrical connections that also render the device nonfunctional. Because of nonuniformities that exist in the product wafers and the highly specialized DRIE equipment. it is not unusual to have both overetched and underetched devices on wafers processed by DRIE. The interdigitated fingers of Rich""s MEMS can overetch while other regions of the wafer are being etched to completion. The cause of this overetch is believed to be that, once the trenches that delineate the fingers breach the underlying cavity, the etch starts to degrade both the sides (lateral erosion) and the backside of the proof mass fingers. This phenomenon is due in part to etch lag, which as used herein refers to the reduced etch rates that are observed for narrower trenches in comparison to wider trenches. As a result, larger parasitic gaps that separate adjacent pairs of capacitively coupled fingers etch faster than the smaller capacitive gaps between paired fingers. The fact that there can be underetched and overetched die on a wafer indicates that the DRIE process window is smaller than is desirable for producing intricate MEMS devices such as Rich""s.
The same etch lag and erosion phenomenon noted for Rich""s MEMS device is believed to occur with essentially any suspended feature DRIE etched from a substrate above a cavity. Consequently, though the DRIE process has the capability of performing deep, high aspect ratio anisotropic etches in silicon and polysilicon, the etch lag and erosion phenomenon associated with the DRIE process complicates the ability to utilize the DRIE process in the micromachining of essentially any suspended feature (e.g., cantilever, bridge, proof mass, finger, tether, etc.) used in a wide variety of devices, such as actuators and passive circuit elements, in addition to linear and rotational motion and acceleration sensors.
The present invention provides a process for forming a microelectromechanical system (MEMS) device by a deep reactive ion etching (DRIE) process during which a substrate overlying a cavity is etched to form trenches that breach the cavity to delineate MEMS structures, including suspended elements. A particular example is the fabrication with a DRIE process of a semiconductor MEMS device used to sense motion or acceleration, and therefore includes a proof mass suspended above a cavity so as to have an axis of rotation perpendicular to the plane of the proof mass, as taught by Rich, Sparks and Zarabadi et al. While the invention will be discussed in reference to such MEMS devices, the invention is applicable to essentially any structure that can be fabricated by forming a trench in a substrate overlying a cavity.
According to the invention, the isolation of structures during the DRIE process accelerates the etch rate of such structures, possibly due to electrochemical and/or thermal influences. The present invention is directed to eliminating heat and/or charge accumulation on the structures in order to minimize or prevent an accelerated etch. As a result, DRIE processing in accordance with the present invention increases the process window by allowing slower-etching structures (e.g., structures delineated at least in part by a relatively narrower trench or trenches) to be etched to completion without overetching more rapidly etched structures (e.g., structures delineated with relatively wider trenches). The invention makes possible the use of a more reliable etch time that can account for variations that exist across a given wafer, between wafers within a lot, and between lots.
The present invention achieves the above objects with a process that entails forming a microelectromechanical device by DRIE etching a semiconductor layer having a first portion that surrounds a second portion suspended over a cavity. The DRIE process forms trenches in the semiconductor layer, some of which breach the cavity to delineate structures from the second portion of the semiconductor layer. A first of the structures remains physically in contact with the first portion of the semiconductor layer, while one or more trenches physically separate a second structure from the first structure and the first portion of the semiconductor layer. Consequently, the first structure is electrically and thermally coupled to the bulk of the semiconductor layer, while the second structure is relatively electrically and thermally isolated from the bulk of the semiconductor layer by the trenches. The invention is based on the determination that electrical and thermal isolation of a structure leads to accelerated etching of the isolated structures while other slower-etching structures are etched to completion, and as a solution provides conductive means that electrically and thermally interconnect the otherwise isolated structure (i.e., the second structure delineated by the trenches from the second portion of the semiconductor layer) with the remaining bulk (i.e., the first portion) of the semiconductor layer.
Consequently, during DRIE etching of the semiconductor layer to form the trenches that delineate the structures, heat and charge accumulation that would otherwise occur on the second structure relative to the first structure is minimized as a result of the conductive means maintaining the first and second portions of the semiconductor layer at a substantially common potential and as a result of the conductive means conducting heat from the second portion to the first portion of the semiconductor layer. After the DRIE etch. or as a result of the DRIE etch. the conductive means is interrupted (e.g. severed) to prevent undesired shorting that would otherwise occur through the conductive means.
In view of the above, it can be seen that the present invention provides a DRIE etching process by which structures of desired geometries can be more reliably and precisely formed. As a result, the present invention is able to take advantage of the deep etching capability of the DRIE process, while compensating for etch idiosyncrasies that adversely affect the structural integrity and durability of a MEMS device, so as to improve yields and device reliability.
Other objects and advantages of this invention will be better appreciated from the following detailed description.